It has passed almost 20 years since an issue of global warming was presented, and the necessity of some measures therefor has been widely recognized from international and academic viewpoints. Among various factors of the global warming, greenhouse gas can be artificially reduced. One of the ideas to achieve the greenhouse gas reduction is efficient use of energy. It is conceivable that the energy use efficiency can be enhanced by using energy without waste that is energy saving and by storing the energy that has not been used.
Particularly, in recent years, which are referred to as “electricity-dependent society”, products that are driven with electric power have been on the rise in industries ranging from home electronics, electronic device to automobiles, and therefore developments of mechanisms that can be driven with less electric power have been focused. Stand-alone type power generators as typified by solar photovoltaic power generation independent on power networks managed by conventional electric power companies have been experimented in a large scale and some of them have been commercialized. As the results, electricity-dependency is more likely to increase more than ever.
While the aforesaid electric utilization devices are driven, electricity (electric power) is constantly consumed. However, upon halt of driving, the electric power becomes surplus but is currently wasted without being stored. For the above-described efficient energy use should be enhanced not only by using a secondary battery for driving devices when the devices are carried or used in the outdoors but also by storing and using the wasted energy.
In the process of technological development or partial commercialization of environment-friendly electric-powered vehicles and hybrid vehicles or solar photovoltaic power generation or wind force power generation placed as power generation with renewable energy, as concrete measures for prevention of the global warming, the electric power sources for these applications have been required to be large in size and capacity. As such power sources, automobile storage batteries/stationary storage batteries have been demanded, which are excellent in input-output characteristics such that they can absorb rapid output fluctuation caused by boost charge/regeneration upon sudden braking in an automobile and natural phenomena.
As described above, the demand of large scale secondary batteries has been increasing and technologies for ensuring safety for the batteries are important in order to spread them to the society. There are various methods for ensuring safety, among which are known to be accompanied with mechanical operations and to utilize chemical reactions. As one of such methods, it is known that the safety can be ensured by an overcharge protection function obtained using some type of an organic compound mixed with an electrolyte to utilize the electrochemical reactivity thereof even when a control circuit is damaged.
This method is disclosed in Japanese Patent No. 3061756 (Patent Literature 1), Japanese Patent No. 3061759 (Patent Literature 2) and the like. This function provides a protection for a secondary battery against overcharge, wherein the compound electrochemically oxidized when exposed to a high potential and undergoes to collapse of its structure, involved with behaviors such as increase in molecular weight (polymerized) or lowered in molecular weight (gasified), during which the pressure-sensing overcurrent breaker equipped in the battery secondarily actuates to physically disconnect the terminals thereby shutting the charging current (Patent Literature 2).
The aforesaid function is a nature derived from the structure composed of the elements constituting the compound and thus is affected in reactivity by a slight difference in the structure.
When the compound is mixed with an electrolyte, it must not adversely affect the functions of the electrolyte and furthermore the characteristics of the secondary battery. However, it is known that the output characteristics or low temperature discharge characteristics of the secondary battery are adversely affected by physical properties such as viscosity and melting point of the compound depending on the type thereof.
That is, it is important for the compound to have a function selectivity that exhibits only a function of protection against overcharge as expected but does not adversely affect the storage battery characteristics. Currently, there are not so many compounds which are really practical in terms of such a function.
Among phenyl-R-phenyl compounds (R is an aliphatic hydrocarbon) described in Japanese Patent No. 3942134 (Patent Literature 3), diphenylmethane, 1,2-diphenylethane and 2,2-diphenylpropane used in the examples are suitable compounds, and 2,2-diphenylpropane is particularly suitable.
However, these compounds must be deemed insufficient in practical characteristics. Specifically, diphenylmethane, 1,2-diphenylethane and 2,2-diphenylpropane have higher melting points and when mixed with an electrolyte, increase the viscosity thereof and are likely to hinder the migration of lithium ions and furthermore have a strong possibility of adversely affecting the output characteristics, low temperature discharge characteristics and the like. As for diphenylmethane, the withstanding voltage measured using a practical electrode (LiCoO2 cathode), the voltage is higher than the normal operation voltage of the secondary battery (4.2 V or lower). However, since the difference is small, the secondary battery in use is likely to be degraded when exposed to an unexpected high voltage upon generation of overvoltage and would be possibly degraded in reliability.
As described above, when diphenylmethane, 1,2-diphenylethane and 2,2-diphenylpropane which are granted for patent as specific compounds represented by a general formula of phenyl-R-phenyl (R is an aliphatic hydrocarbon) are used in a secondary battery, it is difficult to obtain more satisfactory practical characteristics for the battery.